Carbenoid chemistry

The use of rhodium(II) acetate in carbenoid chemistry has also been extended to promoting intramolecular C/H insertion reactions of ketocarbenoids 277,280,280 ,). From the a-diazo-P-ketoester 305, highly functionalized cyclopentane 306 could thus be constructed in acceptable yields by regiospecific insertion into an unactivated... 

It is not known whether there is any carbenoid chemistry of end-on coordinated manganese-diazoalkane complexes 420 410). However, it is known that diaryldiazo-methanes 408,411 and azibenzil411 yield carbene rather than diazoalkane complexes with (n5-C5H5)Mn(CO),THF. 

Du Bois originally used rhodium(n) acetate and rhodium triphenylacetate (tpa) as catalysts and found that regio-and diastereocontrol was influenced by the catalysts, but neither was particularly effective when low catalyst loadings were used. Inspired by the bridged dirhodium catalysts which have been developed for carbenoid chemistry,40,273,274 a second generation catalyst Rh2(esp)2 116 (esp = a,a,a, o -tetramethyl-l,3-benzenedipropionate) was designed which was capable of much higher turnover numbers (Scheme ll).275 Furthermore, this catalyst was effective in intermolecular reactions. 

Coupling of the diazolactams 1382 and 1383 with 2,2 -bisindole (1384) in the presence of catalytic amounts of Rh2(OAc)4 and degassed pinacolone provided staurosporinone (293) directly in 25% yield, and the protected aglycon 1381 in 62% yield. This annulation is believed to proceed via the intermediates 1391, 1392 and 1393, 1394. The degassed pinacolone is both a good solvent for the 2,2 -bisindole substrate and compatible with the carbenoid chemistry (773,774) (Scheme 5.235). 

The total synthesis of the antifungal alkaloid K252a has been reported in which the indolocarbazole nucleus is constructed using novel rhodium carbenoid chemistry <1995JA10413, 1997JA9641>. Thus, reaction of 2,2 -biindole with diazolactam 173 in the presence of rhodium acetate in degassed pinacolone produces indolocarbazoles in moderate yields (Equation 107). 

The [2+1] cycloaddition between metal carbenoid intermediates and alkenes is a very powerful method for the stereoselective synthesis of cyclopropanes [1-3]. Indeed, the vast majority of chiral catalysts developed for carbenoid chemistry were specifically designed for asymmetric cyclopropanation [1-3]. In recent years, however, a number of other enantioselective cydoadditions have been reported. 

Scheme 8 Further attempts at the construction of the cyclohepta[cd]indole system using rhodium carbenoid chemistry...

V K. Singh, A. DattaGupta, G. Sekar, Catalytic Enantioselective Cyclopropanation of Olefins Using Carbenoid Chemistry, Synthesis 1997, 137—149. 

The synthetic potential of silicon substituents in organic and organometallic chemistry has by far not been fully exploited, which is evidenced by the numerous contributions in this chapter. This is examplified for the description of the silyl group as a substituent and as a functional group in carbene and carbenoid chemistry, for the function of the trimethylsilyl substituent in the synthesis of low-valent compounds containing elements ofgroup 15 andfor the influence of a supersilyl ligand to a phosphorous center. 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

Failure to use 2,4,6-Triisopropylbenzenesulfonyl Azide results in substantial diazo imide formation. However, optimization for the formation of the a-diazo imide compounds can be achieved with NaHMDS and p-nitrobenzenesulfonyl azide, followed by a neutral quench (eq 27). These diazo compounds, however, have failed to demonstrate utility in asymmetric carbenoid chemistry. ... 

Another application of rhodium carbenoid chemistry relates to the synthesis of strained-ring nitro compounds as high energy-density materials. Nitrocyclo-propanes are the simplest members of this class of compounds and catalyzed additions of a nitrocarbene to an olefin have only been described recently [40], Detailed studies have shown that the success of the reaction is, as expected, dependent on both the alkene and the nitrodiazo precursor. Consistently with the electrophilic character of rhodium carbenoids, only electron-rich alkenes are cyclopropanated. The reaction has been extended to the synthesis of nitrocyclo-propenes but the yields are good for terminal acetylenes only [41]. 

Donor/acceptor carbenoids 24 were a relatively late arrival to the field of carbenoid chemistry, but have greatly expanded its scope [3], The addition of the electronreleasing moiety, although decreasing the reactivity of the carbenoid, has been found to enhance greatly the selectivity of a variety of transformations, including C-H insertion. Davies and coworkers found that aryl, vinyl, and, to a lesser extent, alkynyl groups are all suitable donors for these reactions [20],... 

An important utility of the Krapcho reaction is not necessarily the decarboxylation step itself. Rather, the fact that the decarboxylation can be made to occur allows several reactions that require malonates or their derivatives to find general synthetic utility. For example, elegant work in the area of rhodium carbenoid chemistry relies on diazomalonates to generate the carbenoid. As utilized by Wee,20 diazomalonate 14 is treated with Rh20Ac4 to generate the carbenoid which inserts into the stereochemically defined tertiary C—H bond. The reaction proceeds exclusively with retention of configuration in forming the new quaternary carbon stereocenter. Decarboxylation of 15 under Krapcho s conditions provides lactone 16, a key intermediate in the synthesis of (-)-ebumamonine. 

This section has clearly demonstrated that enantioselective rhodium-carbenoid reactions only developed strongly in recent years, but it seems that today (1995), they are at the frontier of scientific activities on diazo-carbenoid chemistry. There, I see... 

Diazo carbenoid chemistry, however, is not yet on the finishing line, but developments such as the synthesis of polymer-bound soluble rhodium complexes that are recoverable and, therefore, less sensitive to the exceedingly expensive rhodium (Doyle et al., 1992 a) or the use of relatively inexpensive chiral auxiliaries for enantiocontrol (Davies and Cantrell, 1991) are still energy reserves that may be important to win the race ... 

The synthetic strategy employs novel rhodium carbenoid chemistry in the construction of both the indolocarbazole aglycone 2 and the carbohydrate moiety 3 <97JA9641>. For the total synthesis of pyranosylated indolocarbazoles ((+)-RK286c, (+)-MLR-52, (+)-stauroporine, (-)-TAN-10309) see <97JA9652>. The total synthesis of 4, an enantiomer of the marine furanocembrane rubifolide, starting with an allene was described <97JOC4313>. 

A type of 1,3-dipole that has received considerable recent interest is the carbonyl ylide. One method for its formation makes use of carbenoid chemistry (see Section 4.2). Cyclization of an electrophiUc rhodium carbenoid onto a nearby carbonyl group provides access to the carbonyl ylide. Cycloaddition with an alkyne or alkene dipolarophile then gives the dihydro- or tetrahydrofuran product. For example, the carbonyl ylide 235, formed from the diazo compound 234 and rhodium(II) acetate, reacts with dimethyl acetylenedicarboxylate to give the bridged dihydrofuran 236 (3.148). 

The Carbenoid Chemistry of Lithiated Epoxides C-H insertion and 1,2-Shifts... 

G. Maas, Carbene and Carbenoid Chemistry of Silyldiazoacetic Esters The Silyl Group as a Substituent and a Functional Group , in Organosiiicon Chemistry II From Molecules to Materials (Eds. N. Auner, J. Weis), VCH, Weinheim, 1996, pp 149-159. 

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